Kinetics and Energetics of Electron Transfer to Dimer Radical Cations

Abstract

Spectra of the dimer cations naphthalene (Nap2•+) and ethene (Ethene2•+) were measured in liquid dichloromethane (DCM). The spectra peak at very different energies, 1.2 and 3.3 eV. In DCM dimerization stabilizes Nap2•+ by ΔGd°(Nap2•+) = −218 meV relative to the monomer Nap•+ as determined from the dimerization equilibrium constant. Both dimers can transfer a positive charge to hole acceptor molecules, but for both the rate constants rise more gradually with reaction energetics than do many charge transfer reactions previously studied. A striking observation finds that the rate constant for hole transfer from the Nap2•+ dimer to phenanthrene is smaller by two decades than that from biphenyl•+ monomer to Nap, although both reactions have the same −ΔG° = 0.05 eV. A plausible interpretation for these observations is the presence of an energy of reorganization, λ(M2), for the dimer that involves movement apart of the two partners in the dimer. While the dimerization equilibrium cannot be measured for Ethene2•+, the charge transfer data imply that both ΔGd°(Ethene2•+) and λ(Ethene2•+) are considerably larger, perhaps by factors of 2–4 than for Nap2•+